A typical electroluminescent element has a planar transparent substrate 1, and a first electrode layer 2, a light-emitting layer 3 and a second electrode layer 4 that are layered in this order on the lower side of the planar transparent substrate 1, as shown in FIG. 11. Upon the application of a voltage between the first electrode layer 2 and the second electrode layer 4, the light-emitting layer 3 generates light. The light generated in the light-emitting layer 3 passes through the first electrode layer 2 and the transparent substrate 1, and is emitted upward. Serious problems associated with effective use of the light generated in the light-emitting layer 3 are the transparency of the first electrode layer 2 and the transparent substrate 1, as well as the reflection loss between the first electrode layer 2 and the transparent substrate 1 and that between the transparent substrate 1 and the outside, which is typically air, of the electroluminescent element. When light is emitted out of the transparent substrate 1 to the outside, some portions of the light is totally reflected at the interface between the transparent substrate 1 and the air due to their difference in refractive index, which lowers the efficiency at which the light generated in the light-emitting layer 3 is emitted outward from the transparent substrate 1 located above the layer 3, or the efficiency at which the light is out-coupled from the element.
Patent Documents 1 and 2 teach the formation of small lens arrays in the interfacial surface of the transparent substrate to improve the efficiency at which the light is out-coupled. They also teach that compared with the case where light rays generated in the light-emitting layer 3 come to a smooth surface of the transparent substrate at various incidence angles and go out thereof, the small lens arrays formed in the surface of the transparent substrate make the incidence angles smaller. The smaller incidence angles, in turn, minimize the reflection loss when the rays are emitted to the outside, and improve the efficiency at which the light is out-coupled.
Patent Document 1: JP 9-73983 (1997), A
Patent Document 2: JP 2003-59641, A